Process for preparing a planographic printing master

ABSTRACT

A PLANOGRAPHIC PRINTING PLATE OBTAINING BY XEROGRAPHIC EXPOSURE AND DEVELOPMENT IS HYDROPHILIZED WITH AN AQUEOUS SOLUTION CONTAINING THE PARTIAL ESTE OF A PHOSPHORUS OXY ACID WITH A POLYHYDROXY LOWER ALIPHATIC COMPOUND OR A POLYHYDROXY CYCLOHEXANE.

United States Patent US. Cl. 96-1 7 Claims ABSTRACT OF THE DISCLOSURE A planographic printing plate obtaining by xerographic exposure and development is hydrophilized with an aqueous solution containing the partial ester of a phosphorus oxy acid with a polyhydroxy lower aliphatic compound or a polyhydroxy cyclohexane.

This invention relates to a process for the preparation of a planographic printing plate, and to the printing plate obtained therewith.

Planographic printing is based on the physical property of repellence of greasy materials for water. The printing surface, which is substantially fiat, contains the pattern of the image to be printed in terms of a differentiation in water-repellency. In the ordinary lithographic or planographic printing processes a printing plate is prepared by affixing to a water-attractive, hydrophilic surface, a water-repellent, hydrophobic image, usually greasy, resinous or waxy in nature.

There are several methods known for preparing a lithographic printing plate. According to one of them use is made of a photoconductive insulating material such as an electrophotographic recording layer containing photoconductive zinc oxide dispersed in an electrically insulating binder. To prepare the printing surface said layer is electrostatically charged and image-wise exposed.

The electrostatic image according to the electrical sign of the electrostatically attractable developer particles in respect of the applied charge may be developed as a negative or a positive of the original to be reproduced. According to a common technique the electrostatic image is rendered visible by applying a developer powder, which is held electrostatically to the charged areas of the sheet. The powder image preferably composed of hydrophobic fusible powder particles may be fixed by heating. Normally the difference in hydrophobicity between the developed image parts and the non-covered areas of the recording layer is not high enough for high quality planographic printing and has to be increased. Therefore, it has been proposed to chemically treat the recording layer in the undeveloped areas in order to make the said areas highly water-receptive.

It is known from the United States patent specification 2,952,536 to image-wise hydrophilize a photoconductive insulating recording layer comprising an electrically insulating binder having suspended therein a finely divided photoconductive pigment by means of an aqueous solution providing hydrophilicanions forming insoluble compounds with said pigment, and which anions are provided by a solid organic acid at least as strong as acetic acid.

It has been found now that a photoconductive insulating recording layer comprising an electrically insulating binder having suspended therein an inorganic photoconductive substance, which can provide zinc ions such as photoconductive zinc oxide can be provided with improved hydrophilicity by treating it with an oxyacid of phosphorus partially esterified with a polyhydroxy com- 3,592,640 Patented July 13, 1971 pound, preferably an acid ester of orthophosphoric acid or pyrophosphorie acid and a polyhydroxy compound, preferably an aliphatic or cycloaliphatic polyhydroxy compound.

Partial esters falling within the scope of that definition are described in the published Dutch Patent application 6508534.

Preferably used compounds correspond to the following structural formulae:

The mentioned ester compounds are water-soluble, form a hydrophilic zinc salt and have a hydrophilization power directly proportional to their concentration in the treating liquid.

According to a preferred embodiment the image-wise hydrophilization is effected by treating the undeveloped portions of the recording layer, which are not covered by the hydrophobic developing substance, with an aqueous liquid containing the said partial esters in a concentration comprised between 1 and 25 g. per liter.

Obviously the partial esters used in the present invention may be combined with other known zinc oxide hydrophilizing compounds such as e.g. water-soluble ferricyanides and water-soluble ferrocyanides, mineral and organic acids forming a hydrophilic Zinc salt precipitate in aqueous medium e.g. orthophosphoric acid and watersoluble salts thereof, citric acid and tartaric acid, and the hydrophilizing compounds described in the U.K. Patent applications 10,475/ 67 and 10,476/67.

The resistance to wear of the hydrophilized portions of the recording layer, which resistance is important for obtaining printing plates suitable for long runs, can be increased by the use in combination with the ester compounds of hydrophilic colloids, which may be applied together or after the treatment with said compounds. Suitable hydrophilic colloids for that purpose are e.g. gum arabic, alginic acid, water-soluble alginates e.g. ammonium alginate, caseinates, gelatin, polyacrylic acid esters, polystyrene sulphonic acid, polyvinyl alcohol, and carboxymethylcellulose. A treatment with hydrophilic precipitating or hardening agents for these colloids has proved to be advantageous. So use can be made of formaldehyde as a protein hardener and of heavy metal salts forming a precipitate e.g. with alginates.

In consequence of their important hydrophilic character the zinc salts produced according to the present invention keep the printing properties of the planographic plate intact even after month of storage. Of course, hygroscopic substances e.g. glycerol can be combined with these salts to further increase the hygroscopic character.

The hydrophilizing action of the hydrophilizing agent may be improved by a pre-treatment of the developed and 3 fixed recording layer with a softening or swelling agent for the binder, occasionally in combination with a Wetting agent that improves the penetrating powder of the hydrophilizing agent in the recording layer. In this way much more zinc oxide grains are reached by the hydrophilizing agent and thus hydrophilized.

As swelling agents preferably water-miscible compounds such as acetone and methyl ethyl ketone are used.

In order to set free zinc ions from the zinc oxide photoconductor, the pH of the hydrophilizing liquid preferably varies between 3 and 6.

According to a preferred embodiment the electrophotographic recording layer is rendered water-receptive at the areas to be hydrophilized after the printing master has been mounted on the press, thus avoiding any separate immersion treatment. The hydrophilizing treatment of the said layer may be carried out by means of an absorbent pad impregnated with an aqueous solution containing the ester compound.

In order to improve the reproducibility of the printing quality, the partially esterified oxy acid compounds of phosphorus are added to the aqueous phase, which is applied during printing separately or together with the lipophilic phase of a planographic printing ink. Thus, e.g. these ester compounds can be added to the aqueous liquid, which is supplied by the damping system to the planographic plate; or can be incorporated into the emulsified aqueous phase of a planographic emulsion type ink.

Electrophotographic recording materials especially suited for use in the preparation of a planographic printing plate are e.g. described in the published Dutch patent applications Nos. 6608814 and 6608815.

Any known process for forming the electrostatic latent image and hydrophobic image can be used.

According to a common technique the hydrophobic image is formed by the subsequent steps of producing an electrostatic image on a photoconductive zinc oxide/hydrophobic binder layer by integrally electrostatically charging that layer, then image-wise exposing and developing the latter layer with a hydrophobic developer powder, which is fixed to the recording layer e.g. by heating.

The powder image can be formed by the known dry carrier-toner development or by a liquid development based on electrophoresis wherein charged hydrophobic particles are attracted from an electrically insulating liquid to the charged areas of the recording layer. Such development technique is e.g. described in the UK. patent specification 755,486.

According to a developing technique described in the United Kingdom patent specifications 987,766; 1,020,505; 1,020,503; 1,033,419 and 1,033,420 an aqueous liquid is image wise applied to an electrostatic charge pattern present in a recording layer containing photoconductive zinc oxide. Said aqueous liquid can be an aqueous dispersion of hydrophobic material, which after evaporation of the aqueous liquid forms a hydrophobic deposit in accordance with the printing parts.

The following examples illustrate the present invention Without, however, limiting it thereto.

EXAMPLE 1 A layer of photoconductive material consisting of photoconductive zinc oxide dispersed in an insulating binder (3 parts by weight of zinc oxide to one part by weight of resin binder) was coated on a sheet of aluminium foil laminated to a paper support. The resin binder contained on a weight basis 80% of styrene-butadiene copolymer (80/20), 20% of silicone resin (Silicone resin SR-82 supplied by General Electric-Silicone Products Department, Waterford, N.Y., U.S.A.). The recording layer was charged with a negative corona with a tension of 6000 v. on the corona wires and exposed through a graphic original. Development was carried out by using the magnetic brush technique with iron particles as carrier and gilsonite as fusible toner. The powder image was fixed to the layer by heating for 90 seconds at 140 C.

4 A cotton pad was impregnated with a solution containing the following ingredients:

Phytic acid 10 Water The plate was rubbed slightly with the cotton pad while on the press. The quality of the prints obtained was very good.

When using 0.3% by weight of phytic acid in the liquid of a damping system of a planographic printing device, up to 5000 excellent sharp copies were obtained.

EXAMPLE 2 Example 1 was repeated, with the proviso, however, that the ester compound No. 1 of the table was used. The quality of the plate was fully equivalent to that of Example 1.

EXAMPLE 3 A layer of photoconductive material consisting of photoconductive zinc oxide dispersed in an insulating binder (6 parts by weight of zinc oxide to one part by weight of binder) was coated on a flexible aluminium foil.

The binder was prepared from a mixture of EPOK X-l772 (ammoniacal alkyd resin salt marketed as a 66 to 68% (by weight) aqueous solution by British Resin Products) and EPOK W9801 (a 72% to 75% aqueous solution of a melamineformaldehyde resin marketed by British Resin Products). The coated layer was dried for 24 h. at room temperature and cured for 1 min. at C.

The cured recording layer was charged with a negative corona with a tension of -6000 v. on the corona-wires and exposed to a line-copy.

The electrostatic image was electrophoretically developed by means of an electrophoretic developer prepared by diluting the hereinafter described concentrated developer composition in a ratio of 15/1000 by means of the hydrocarbon solvent Shellsol T (trade name):

Carbon black (average particle size: 20 m 30 g. Zinc trideeyl phosphate (dispersing agent): 1.5 g. Shellsol T (trade name): 750 ccs.

Resin solution prepared as described hereinafter: 150 g.

The resin solution was prepared by heating at 60 C. 500 g. of Alkydal L 67 (trade name of Farbenfabriken Bayer AG, Leverkusen, W. Germany, for a linseed oil (67% by weight) modified alkyd resin) and 500 ccs. of white spirit containing 11% by Weight of aromatic compounds, till a clear solution was obtained.

Hydrophilization of the developed plate was carried out as described in Example 1. When printing with a fatty lithographic ink very sharp prints were obtained.

EXAMPLE 4 A layer of photoconductive material consisting of photoconductive zinc oxide dispersed in a ratio of 8 to 1% by weight in an alkyd resin as binding agent was coated on a baryta-coated paper of g./m. Said recording layer was used for the powder development described in Example 1 and for the electrophoretic development described in Example 3. Hydrophilization after development was carried out as described in Example 1. With the obtained planographic printing master 500 high quality prints were produced.

What we claim is:

1. In a process for preparing a planographic printing plate comprising the steps of forming a latent electrostatic image on a photoconductive insulating recording layer containing an inorganic photoconductive zinc oxide, developing said image with a toner forming a hydrophobic deposit on the image areas of said layer, the improvement of treating the portions of the recording layer which are not covered with said hydrophobic deposit with an aqueous solution containing an oxy acid of phosphorus partially esterified with a polyhydroxy lower aliphatic compound or a polyhydroxy cyclohexane.

2. Process for preparing a planographic printing plate according to claim 1, wherein said oxy acid 'of phosphorus is orthophosphoric acid or pyrophosphoric acid.

3.;Process for preparing a planographic printing plate according to claim 1, wherein the said ester is applied to the printing plate after mounting the printing plate on a printing press.

4. Process for preparing a planographic printing plate according to claim 3, wherein the said ester is applied in the aqueous liquid of a damping system of a planographic printing device.

5. Process for preparing a planographic printing plate according to claim 3, wherein the said ester is applied in the aqueous phase of a planographic printing ink cornposed of a lipophilic phase dispersed in a hydrophilic 15 phase.

6. Process for preparing a planographic printing plate according to claim 1, wherein the said ester is used in 6 dissolved form in an aqueous liquid in a concentration of l to 25 g. per liter.

7. Process for preparing a planographic printing plate according to claim 1, wherein said aqueous solution has a pH of about 3-6.

References Cited UNITED STATES PATENTS 3,307,951 3/1967 Adams et al 9633X 3,315,599 4/1967 Lind 96--1X 3,323,451 6/1967- Casey et a1. 9633X 3,386,850 6/1968 Adams et al 9633X GEORGE F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 96-33; 101-465 

